Accretion (astrophysics)

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In astrophysics, the term accretion is used for at least two distinct processes.

The first and most common is the growth of a massive object by gravitationally attracting more matter, typically gaseous matter in an accretion disc. Accretion discs are common around smaller stars or stellar remnants in a close binary, or black holes in the centers of spiral galaxies. Some dynamics in the disc are necessary to allow orbiting gas to lose angular momentum and fall onto the central massive object.

The second process is somewhat analogous to the one in atmospheric science. In the nebular theory, accretion refers to the collision and sticking of cooled microscopic dust and ice particles electrostatically, in protoplanetary discs and Gas giant protoplanet systems, eventually leading to planetesimals which gravitationally accrete more small particles and other planetesimals.

Use of the term accretion disc for the protoplanetary disc thus leads to confusion over the planetary accretion process, although in many cases it may well be that both accretion processes are happening simultaneously (e.g. T Tauri).

The Jovian protoplanets probably have discs of their own, in close analogy to the solar system as a whole. The Jovian protoplanet may be accreting gas from its surrounding disc in the first process, at the same time that dust and ice particles in the disc are accreting into moonlets and ring systems, in the second process.